Device for current control and discharge of transient overvoltages



Nov. 24, 1942. o. HUTCHINS ETAL 2,393,080

DEVICE FOR CURRENT CONTROL AND DISCHARGE OF TRANSIENTv OVERVOL TAGES Original Filed Sept. 2:5, 1935 m a n .h

mhmmwmmh mm 5 wagrauoww w w w a yadwmaw a mflQ Patented Nov. 24, 1942 DEVICE FOR CURRENT CONTROL AND DIS- CHARGE or TRANSIENT ovERvoL'rAGEs Otis Hutchins, Niagara Falls, N. Y1. and John Robert McFarlin, Philadelphia, Pa., assignors,

mesne assignments, to Electric Service Supplies Company, Philadelphia, Pa., a'corporation of Pennsylvania Original application September 23,1935, No. 41,834, now Patent No. 2,150,167,

Serial dated March 14, 1939. Divided and this application March 11, 1939, Serial No. 261,290

Claims.

'Our invention relates to improvements in methods and devices for discharging or equalizing excessive electrical potentials for protective purposes and for controlling the flow of electricity, which utilize a characteristic element or material having a rapid decrease in resistance when subjected to increasing voltages, and providing what is commonly known as a valve action.

Our improvements are applicable to a wide variety of apparatus used in connectionjwith the production, transmission and use of electricity and particularly in connection with lightning arresters, high potential dischargers, voltage equalizers, surge absorbers, resistors, rectifiers, oscillation damping elements, current isolators, voltage subdividers, voltage reducers and electrothermal elements.

We have discovered that the variations in the resistivity of masses of crystalline valve action granules with variations in voltage is accelerated by the presence of oxides, and particularly of silica, on the surfaces of thegranulesparticu- -cessive electrical potentials than the oxidized silicon carbide, having a dull, mat, porous or powdery surface, claimed our Patent No. 2,150,167, but may be more advantageously used for certain. electrical purposes than commercially pure silicon carbide crystals heretofore used. The silicon carbide is preferably oxidized in the formof discrete'or non-agglomerated crystals, andmay be used in such form in accordance with "our invention as discrete grains or granules closely compacted under pressure in a container, or the grains may be bonded together to form blocks, rods, or other shapes by suitable binders, such as ceramics, organic binders orthe like, and a greatly improved bond is effected by the use of oxidized silicon carbide granules having sla'gged lumps thereon, particularly when such granules are bonded under heavy pressure with a minimum amount of binder.

The primary advantages of our invention result, however, from the great improvement in electrical characteristics of devices embodying silicon carbide granules having oxidized surfaces,

larly when such masses are used in conjunction,

with spark gaps; and that dynamic failure voltage may be increased by the use of such granules in discharge paths provided for discharging and equalizing high electrical potentials.

as compared'with the silicon carbide granules which are substantially or commercially pure. The improved electrical characteristics imparted to theoxidized silicon carbide grains vary with the amount of true silica and free carbon 0n the Our invention involves the control of the flow of electric current by providing therefor a path containing such granules having oxidized surfaces and consisting preferably of crystalline silicon carbide having an integral surface bloom or coating of metallic oxide, principally true silica,

with a low content of carbon. Thebloom or c coating generally contains small amounts of other metallic oxides, such as iron oxide or alumina.

This bloom or coating of silicais formed in-,

tegrally on the crystalline surfaces by the oxidation thereof, preferably by roasting and tumbling the silicon carbide in the presence of an oxidation accelerator, such as iron oxide, lead oxide or nickel oxide, under oxidizing conditions, as set forth in our Letters Patent No. 2,150,167, whereof the present application is a division.

Excessive roasting or accelerator results in the formation of a surface glaze, in the form of surface thereof and the glaze of thesurface.

The changed and improved electrical characteristics of crystalline granules having silicon and the increase in their carbide cores and oxidized surfaces are evidenced by their higher impedance to dynamic currents;

dynamic failure voltage, or the voltage required to maintain a continuous flow of dynamic current through a column of granules generally in series with one or more are gaps, after an initial flow of dynamic current hasbeen initiated by means of an impulse or surge current passed through the granules while the dynamic voltage is connected therewith.

When the silica coating is in the form of a glaze or of slagged lumps, the impedance to surge or impulse currents is increased as compared with the impedance thereto of ordinary a thin film or in the form of slagged lumps,

while maintaining the high surface content of true silica which results from the disintegration or decomposition by oxidation of the surface of the silicon carbide crystals.

viders, etc.

silicon carbide and may be used more advantageous'ly than the latter in isolators, voltage subdi In devices for discharging or equalizing excessive electrical potentials for protective purposes,

- it isimportant that the dynamic failure voltage Such glazed oxidized silicon carbide is less? advantageous for discharging or equalizing exbeas high as possible and in excessof certain predetermined minimum values under given conditions of test appropriate to the particular installation.

Excellent results are obtainable with grain having a true silica content of 1.7% to 1.9%, and with the surface silicon, iron, aluminuman'd free carbon kept as low as possible. The discharge or flow of electric currents may, however, be advantageously controlled or .regulated by providing a conducting path containing-silicon carbide granules having a surface coating of true silica as low as .5% and up to 3% or higher, and with a free carbon content up to 105% 'if the-true silica content is correspondingly high.

Characteristic elements forlig'htning arresters embodying our present invention possess very high resistance to low values. of voltage impressed across the terminals but are relatively goodconductors at high values of voltage impressed across the terminals, and consequently permit currents of abnormal voltage to flow with relatively.-little impedance but offer very high resistance to current tending to flow by reason of normal system voltage impressed across the terminals of the characteristic elements.

By the utilization of our 'improvementsin such an arrester, a minimum quantity of granules may be employed to secure the desired 'arc suppressive effect, and suchgranules impose minimum permissive impedance -'to surge currents combined with the desired impedance to dynamic current and are suppressive effect. The granules'have-a high degree of permanence under normal operating conditions and hold to their original characteristics, so that the efliciency'of the arrester is unaffected over long periods of time, and "the granules exhibit to a very marked degree the inverse change in resistance due-to variations .in voltag impressed across them. By our improvements, the performance of lightning'arresters of like construction are rendered much more .uniform and there is avoide'dithe wide variations in the performance of lightning :arresters .heretofore made embodying characteristic elements, and the causes of which have beenuunascertained.

Our studies haveindicated that wherejthe content of surface silicon approximates oris greater than the content of'true silicaon'ithe'surface,-it has a deleterious effect onthe .arc suppressing capacity of the arrester. 'In silicon -.carbide granules Where'the percentagezo'f silica is in excess of the silicon, and-particularly where it is greatly in excess, slight variations in surface silicon seem to have relativelylittle effect-on the characteristics of the grain for lightning arrester use, but it appears probable that 'if the surface silicon could be .practically eliminated, such elimination would measurably better -.the granules for lightningarrester-use, but it. hasso far been found impossibleto-eliminate the surface silicon and at the same .time secure the desired high percentage of. surface silica.

The presence of-free carbon onthe siliconv carbide granules has been found to lower the impedance to the passage of surge currents but likewise lowers the impedence to dynamic current and lowers the arc suppressive effect of the granules, thereby-necessitating the use of longer column lengths of granules to secure proper lightning arrester operationoncircuits having a given voltage rating. It is therefore deemed preferable to hold to a minimum or eliminate free carbon from the granules used-in practicing our invention.

The amount of true surface silica on siliconcarbide granules very definitely affects the pergiven voltage ratings. found that to obtain the desired value of imformance of such granules as characteristic elements of lightning arresters and other current discharging, voltage, equalizing and control apparatus.

When'the-surface silica is converted into thin iridescent films on the grains, the arc suppressive effect is maintained or improved but the impedance issubstantially increased as compared withgrainszhaving porous surfaces as described in our Patent No.2;l50,167. Consequently, where conditions are such that it is possible to sufficiently decrease the length of the crystal path toobtain desired values of impedance, lightning arresters may be evolved in accordance with the present invention'from glazed grain suitable for Where, however, it is bloom, in accordance-with the. invention of our Patent No. 2,150,167.

The accompanying drawing is a longitudinal sectional view illustrating the embodiment of our invention in a lightning arrester. In the embodiment of our invention illustrated in the drawing, the housing I contains spaced electrodes '2 forming spark gaps in series with the discrete mass 3 of silicon carbide granules having surfaces oxidized and glazed inaccordance with our present invention; thespark gaps being proportioned relatively to the normal voltage 'of the circuit for which the arrester is designed, and the'length :of the crystalline path and the size of the granules being proportioned to produce an'arc suppressive or interrupting efiect proportional to the normal system voltage. The other structural features o'fthe arrester per se form nopart of the present invention and hence are not specifically described.

Having described our invention, we claim:

1. A device for controlling the flow of electric currentincluding a container containing a current-carrying compacted mass of discrete grains ofcrystalline, valve action silicon carbide granules having integral oxidizedglazed surfaces containingoxide, principally silica, on the granules.

2. A device for controlling the flow of electric current and comprising a body of grains having crystallinesilicon carbide cores and having surfaces composed .of disintegrated portions of the crystalline structure converted into oxide consisting principally of silica with small amounts of iron oxideand alumina.

3. A device for discharging or equalizing excessive electrical potentials for protective purposes comprising a discharge path containing a mass or body of granules having integral surface coatings containing silica and silicon; the silica being substantially in excess of' the silicon.

4. A device for controlling the flow of electric current including a current-carrying mass of crystalline granules having glazed oxidized surfaces containing silica integral with the crystalline portions of the granules, said surfaces being free from at least a portion of the carbon normally present in silicon carbide.

5. A device for controlling the flow of electric current including a current-carrying mass of silicon carbide granules having slagged surface coatings composed principally of silica integral with the granules and free from at least a portion of the carbon normally present in silicon carbide.

'6. A device for discharging or equalizing excessive electrical potentials for protective purposes comprising a body of granules having silicon carbide bodies and having slaggedsurfaces containing silica integral with the bodies and devoid of free carbon in excess of .05%.

'7. A device for controlling the flow of electric current comprising a body of grains having silicon carbide bodies and having thereon slagged lumps with a high content of true silica integral with the bodies and allow content of free carbon.

8. A device for controlling the flow of electric current comprising a body of granules having crystalline silicon carbide bodies and surfaces composed of irridescent films consisting primarily of silica free from a portion of the carbon normally present in silicon carbide surfaces.

cessive electrical potentials for protective purposes comprising a body of grains having crystalline silicon carbide bodies and glazed and slagged surfaces having a true silica content integral with i V the bodies of the order of 1.75% of the Weight of the grain and a free carbon content not exceeding .05% of the weight of the grain.

10. A device for discharging or equalizing excessive electrical potentials for protective purposes comprising a mass of silicon carbide bodies I bonded together by a binder and characterized by surface layers having slagged lumps of silica on andintegral with such bodies, said slag lumps interlocking with said binder said surface layers being devoid of at least a portion of the carbon normally present in silicon carbide.

OTIS HUTCHINS. JOHN ROBERT MCFARLIN.

9.- A device for discharging or equalizing ex- 

